1. Field of the Invention
The present invention relates to a process for producing a porous coating layer, especially a porous coating layer having a prescribed pattern, which is suitable for an active material layer of an electrode plate for a secondary battery with a nonaqueous electrolyte; a process for producing an electrode plate for a secondary battery with a nonaqueous electrolyte, in which the above-mentioned process for producing the porous coating layer is utilized; the electrode plate for a secondary battery with a nonaqueous electrolyte obtained by the above-mentioned process; and a sheet for peeling an active material layer, especially a sheet for peeling an active material layer used in the above-mentioned process for producing the electrode plate for a secondary battery with a nonaqueous electrolyte.
2. Description of the Related Art
In recent years, reduction in size and weight of electronic equipment and communication equipment has rapidly been advanced. This advance has also required reduction in size and weight of batteries used as a driving power source for these equipment. As a result, there has been proposed commercialization of secondary batteries with a nonaqueous electrolyte in which lithium ion secondary batteries having high voltage and high energy density were exemplified as an typical example, in replacement of conventional alkaline batteries.
FIG. 22 shows a structure of a cylindrical shape-lithium ion secondary battery as generally used. The cylindrical shape-lithium ion secondary battery 34 as generally used has an external; appearance given by a metallic column. The battery 34 has a positive electrode 35 at its one end, and a negative electrode 36 at its another end. The battery 34 has an inside structure in which a positive electrode plate 37 generally made of lithium compound such as LiCoO.sub.2, a negative electrode plate 38 made of carbonaceous material and a separator 39 arranged between the positive electrode plate 37 and the negative electrode plate 38 for preventing a short circuit of them are wound in a piled up state. The battery is filled with a nonaqueous electrolyte 40 as an electrolyte, which is obtained by dissolving lithium salts into an organic solvent, since lithium contained in the positive electrode plate 37 has a character of reacting with water. Terminals for introducing an electric current (i.e., a positive cable terminal 41 and a negative cable terminal 42) are connected to the above-mentioned electrode.
Regarding an electrode plate, which has a great influence on the performance of the secondary batteries with a nonaqueous electrolyte, there have been proposed methods for increasing the area of the electrode plate which is wound in the battery by reducing the thickness of the electrode in order to prolong the charge-discharge cycle life and to increase the energy density. For example, Japanese Patent Laid-Open Nos. 10456/1988 and 285262/1991 disclose a process for producing a positive electrode plate, which comprises the steps of: dispersing and dissolving an active material powder for a positive electrode plate, which comprises metallic oxides, sulfides, halides and the like, conductive agents and a binder into a suitable wetting agent (hereinafter referred to as the "solvent") to prepare an active material coating composition in the form of paste, and applying the above-mentioned active material coating composition on the surface of a collector as a substrate, made of a metallic foil to prepare an active material layer for a positive electrode plate (hereinafter referred to as the "positive electrode coating layer"). An active material layer for a negative electrode plate obtained by applying the above-mentioned active material coating composition on the substrate is hereinafter referred to as the "negative electrode coating layer". Both of the positive and negative electrode coating layers are simply referred to as the "coating layer". In the preparation of such a positive electrode plate, there is used as a binder, fluororesins such as polyvinylidene fluoride; silicone-acrylic copolymer; styrene-butadiene copolymer, and the like.
A negative electrode plate can be obtained by adding a binder and a suitable wetting agent (i.e., a solvent) to an active material such as carbon for a negative electrode plate to prepare an active material coating composition in the form of paste, and applying the thus prepared active material coating composition to a collector made of a metallic foil.
The binder for preparing the active material coating composition for the above-described coating type electrode plate is required to be chemically stable against the nonaqueous electrolyte, insoluble in the electrolyte, and soluble in a certain solvent to be able to be applied to the surface of the substrate in the form of thin film. The coating layer obtained by applying the coating composition, and drying same is required to be so flexible that there is no occurrence of peeling, chipping and cracks in the coating layer during the assembling step of the battery, and to be excellent in adhesivity to the collector made of a metallic foil.
The existence of the active material and the coating layer is unfavorable for a certain portion of the above-mentioned electrode plate, for example, for a portion to which a terminal for introducing an electric current is connected, and for a portion along which the electrode plate is subjected to a cutting work. Accordingly, the electrode plate has non-coated portions on the basis of a prescribed pattern determined when making a plan for a battery. FIG. 5 is a plan view of the electrode plate. There are formed non-coated portions, i.e., peeled portions 6 to which cable terminals are connected, between the coating layers 2. FIG. 23 is a plan view of a original sheet for a plurality of electrode plates. The original sheet has longitudinal non-coated portions along the both longitudinal edges of the original sheet and lateral non-coated portions 4 which cross the above-mentioned longitudinal non-coated portions at right angles.
As a process for forming a pattern of the non-coated portion, there has been a process of mechanically controlling a coating head during applying the coating composition for an electrode plate onto the collector, to directly form each pattern of the coated portion (i.e., a portion in which the coating layer exists) and the non-coated portion, and another process of peeling the prescribed portion of the coating layer (prepared by applying the coating composition for the electrode plate onto the collector and drying same) by means of a mechanical means, for example, a spatula or the like, to form the non-coated portion.
However, the process of mechanically controlling the coating head has a problem of insufficient mechanical accuracy, thus making it hard to form a prescribed pattern at a high velocity while maintaining a proper measurement accuracy of the pattern, and leading to non-uniform thickness of the coating layer. The process of partially peeling the dried coating layer requires a long period of time for the peeling, and has problems that a proper measurement accuracy of the pattern cannot be maintained, and production of powder at peeled edges of the coating layer on the basis of the pattern may be caused during or after the peeling step.